The invention relates to an actuation device and to an actuation method for a DC-DC converter, in particular for a push-pull forward converter in a full-bridge circuit.
Synchronous rectifier circuits are usually used for DC-DC conversion, for example for supplying power to a low-voltage electrical system of a vehicle, in the case of push-pull forward converters in full-bridge or half-bridge circuits. The power semiconductor switches, for example MOSFETs, used on both the primary and secondary sides for this purpose must be protected against overvoltages or current levels which are too high.
To actuate active synchronous rectifier circuits, pulse-width signals are provided for this purpose by a microprocessor, which signals are converted, for example by a pulse-width-modulation member, into corresponding PWM actuation signals for the individual semiconductor switches. When a push-pull forward converter is disconnected, it can lead to a secondary-side discontinuous current mode (DCM) owing to the drop in the primary-side supply voltage, which discontinuous current mode can damage the secondary-side semiconductor switches. At the same time, it is possible for the magnetic flux reserve during the generation of the PWM actuation signals to reduce and for an excessive magnetizing current to lead to saturation of the primary side of the transformer of the push-pull forward converter and, as a result, to a reduction in the magnetic inductance, which in turn can constitute a risk of damage for the primary-side semiconductor switches owing to high primary-side currents.
The document U.S. Pat. No. 7,561,450 B2 discloses a protective circuit for a DC-decoupled DC-DC converter, which protective circuit can be used to prevent high currents through semiconductor switches of the DC-DC converter when a load demand on the secondary side of the DC-DC converter exceeds a critical threshold. For this purpose, the energy transfer from the primary to the secondary side is limited for a predetermined time period, after which the protective circuit can safely disconnect the DC-DC converter.